(1) Field of the Invention
This invention relates to the deposition of metallic coatings from plating solutions. More particularly, this invention relates to wiping the cathodic coating surface of sheet and strip and during continuous electroplating and more particularly still to the use of a substantially solid wiper blade during such electroplating.
(2) Prior Art
As detailed more particularly in U.S. application Ser. No. 08/179,520 filed Jan. 10, 1994, the disclosure of which is hereby expressly incorporated in and made a part of the present application, a number of coatings are deposited from so-called plating baths subjected to an imposed electrical potential basically enhancing an already naturally occurring tendency for metal ions in the solution to plate out.
Since the coating of a cathodic workpiece is largely merely the acceleration of a naturally occurring process or phenomena, fairly small changes in technique and apparatus accentuating those conditions that favor deposition and de-emphasizing these conditions that disfavor deposition, may have rather large effects upon the final coating obtained. The history of improvements in the field, therefore, is largely one of progressive small improvements and adjustments to improve the conditions for deposition of various coating metals on a metallic substrate temporarily included as the cathode in a plating circuit.
It has been found, for example, by the present inventors as well as others that it is conducive to good coating results to remove the hydrogen bubbles which are produced at the cathodic work surface by transfer of electrons not only to the positive ions of the coating metal in the solution, but also to positive hydrogen ions in the electrolytic solution. The initial cathodic film is believed to be a combination or mixture of both hydrogen ions and atomic or molecular hydrogen. This film initially is only one atom thick. It interferes to some extent with good coating in that it may tend to hold the larger metallic coating ions away from the surface to be coated. However, the hydrogen atoms are small and the layer of hydrogen is initially discontinuous so that their initial interference with coating is not too serious.
If nothing is done to remove the hydrogen from the surface coating during the coating process, coating will usually continue, even though it may be seriously interfered with by the increasing hydrogen present as the thickness of the hydrogen layer increases the interference with efficient plating out of metal atoms upon the substrate surface. Such hydrogen, as it accumulates, however, tends to coalesce into larger local accumulations resulting in small bubbles and then larger and larger bubbles until such bubbles have sufficient volume and buoyancy to overcome their initial attraction for or adhesion to the substrate surface and float upwardly in the solution to the surface where they are finally dissipated into the surrounding atmosphere or local environment. Consequently, the hindrance to coating caused by the presence of hydrogen gas at the surface of a cathodic workpiece does not tend to progress to the limit where it would cut off electrolytic plating completely. However, hydrogen is still a very significant hindrance to rapid coating or plating and the larger bubbles clinging to the surface of a workpiece may even lead to macroscopic pits and other defects in an electrolytic coating.
A second significant problem which has been long recognized in electrolytic coating baths is depletion of the electrolytic solution as coating progresses. In many cases, the only result is that the coating rate slows down as there are progressively less coating metal ions in the solution to plate out. This decreasing coating rate has been counteracted by pumping in fresh coating solution, throwing in chunks of soluble coating metal for solution to "beef up" the electrolyte as well as other expedients. The trend has been for closer and closer control of the electrolyte composition during coating. Sometimes this has been implemented by continuous testing or analysis of the electrolytic bath as coating progresses. In addition, the coating solution baths have been mixed by impellers or the like, force circulated and re-circulated as well as frequently tested to hold them to a desired composition.
It has also been recognized that the coating bath next to a workpiece being coated may become locally depleted of coating metal ions and that such depletion may compromise efficient coating. Some installations have adopted the expedient of forced circulation of electrolyte past the point of coating or through a restricted coating area to increase the efficiency of coating. If the forced circulation is rapid enough, such circulation in itself also tends to detach bubbles of hydrogen from the cathodic coating surface, in effect, "killing two birds with one stone". However, the use of forced circulation of this type by pumps, jets and the like is not only unwieldy and expensive, but is believed by some to possibly have detrimental effects upon the coating itself because of the generalized rapidity of movement between the coating solution and cathodic workpiece, which macroscopically, at least, may interfere with efficient plating out of the metallic ions upon such work surface. Among the processes which have made use of rapid forced circulation is the so-called gap coating process in which a small coating gap between a coating anode and a cathodic workpiece is created and electrolytic solution is forced rapidly through such gap or opening.
Depletion of the coating solution has recently been found by one of the present inventors to be particularly serious in chrome plating solutions in which insoluble electrodes are used. It has been found that unless the chromium plating operation is maintained substantially continuous and at a fairly uniform rate that hard chrome is difficult to efficiently plate out in a brush-type coating operation, or, for that matter, in semi-brush type operations.
While various efforts to remove hydrogen bubbles from the coating surface in an electrolytic coating bath at the point of deposition have been tried, none has provided the ultimate quality of coating and efficiency of the coating operation which has been desired. Likewise, the ultimate in practical prevention of localized depletion in a coating bath has also not been attained.
A further problem in the continuous coating of a flexible material such as sheet, strip and wire products is that the efficiency of electroplating usually increases as the spacing between the electrodes, one of which is the material to be coated, decreases. In other words, the efficiency of coating is usually inversely related to the spacing between the electrodes one of which is the workpiece. However, due to the flexibility of the material being coated, it must, as a practical matter, be held away from the opposing electrode a sufficient distance to prevent arcing between the cathodic work material and the coating electrodes or anodes. The longer the unsupported run of material past the coating electrodes, the more deviation of the flexible material from its intended path is likely to occur, while closer spacing of supporting rolls or the like decreases the area available for coating and interferes with continuous coating. Very close spacing of the coating electrodes and the material being coated has been effected by the so-called jet coating process alluded to previously, but such process is complicated and sensitive to minor changes, making it suitable only for highly sophisticated coating lines.
There has been a need, therefore, for a means for removing hydrogen bubbles and cathodic film from a cathodic coating surface, preventing localized depletion of the coating bath with respect to coating material as well as allowing closer spacing of the coating electrodes and material being coated. The present applicants have found that a very effective means for accomplishing all three of these purposes is by the use of a relatively thin wiping blade applied to the surface of the workpiece at spaced intervals with a light contact. Such wiping blade deviates or strips away from the coating surface the relatively stable surface layer of electrolyte which tends to be drawn along with a moving cathodic surface, mixing and encourages replenishing of the electrolyte next to the cathodic surface. It also at the same time wipes or sweeps away bubbles of hydrogen as well as encourages coalescence of small bubbles and films of hydrogen into large bubbles for subsequent wiping away. In addition, the wiping blade very effectively supports the material being coated, particularly in the case of relatively flexible material, and prevents its deviation from its intended path and, therefore, allows closer spacing of the coating electrodes and the surface of the material being coated. Some of the more pertinent prior art patents related to the above noted problems and their solution are as follows.
U.S. Pat. No. 442,428 issued Dec. 9, 1890 to F. E. Elmore, discloses burnishing of the surface of a product being electroplated by impinging a burnishing material such as agate, bloodstone, flint or glass against the surface being coated during the time coating deposition is proceeding. These substances are characterized by Elmore as being non-conducting substances capable of burnishing and not acted upon by the coating electrolyte.
U.S. Pat. No. 817,419 issued Apr. 10, 1906 to O. Dieffenbach, discloses the use of comminuted kieselguhr in an electrolytic bath to act upon the surface of a workpiece during electrodeposition of metallic coatings. Dieffenbach states that his kieselguhr has an advantage over previously used sand, pumice-stone, brick dust, wood flour, and chaff of being "much harder and sharper edged so that it is capable of cutting up more readily" than the other substances, "the small bubbles of hydrogen that are deposited on the cathode".
U.S. Pat. No. 850,912 issued Apr. 23, 1907 to T. A. Edison, discloses that during the plating of iron, the formation of gas bubbles frequently results in the coating being pitted or even perforated. In order to avoid such pitting by the formation of gas bubbles, Edison introduces a quantity of crushed charcoal into the solution which, he states, "will rub over and scour the surface of the deposited metal to polish the same and wipe off any gas bubbles which may tend to accumulate thereupon".
U.S. Pat. No. 1,051,556 issued Jan. 28, 1913 to S. Consigliere, discloses the use of a number of small, non-conducting bodies such as glass or porcelain balls and pebbles having rounded edges within an electrolytic coating bath, which "burnishing" bodies roll and beat on the surface of the body or "mold" upon which the metallic layer is being deposited or has already been deposited while the electric current is turned on.
U.S. Pat. 1,236,438 issued Aug. 14, 1917 to N. Huggins discloses an apparatus for densifying electrodeposited material in which a roller positioned above the surface of the coating bath impinges upon the surface of a round body being coated as such body rotates out of the bath and wherein the surface is electroplated as the body rotates again down into the bath. Huggins states that for various reasons still undiscovered, but with which most of those skilled in the art are familiar, the metal deposited by the electrolytic bath is frequently spongy and unevenly deposited and his apparatus consolidates it.
U.S. Pat. No. 2,473,290 issued Jun. 14, 1949 to G. E. Millard discloses an electroplating apparatus for plating crankshafts and the like with chromium in which a curved anode partially surrounds the portion of the workpiece to be coated. The curved anode has orifices in its surfaces to allow the escape of bubbles formed during the coating process and also has extending through its surface, a support for a so-called positioning block or scraper block 54 which is provided to maintain a close spacing between the anode and cathodic workpiece. Millard states also that his spacing block removes gas bubbles from the cathode and also removes threads of chromium. He also states that the block, which has a significant width along the line of coating, dresses and polishes the cathode during plating. The aim of Millard, is clearly to burnish or compact the coating surface somewhat in the manner of the earlier Huggins patent. While Millard talks, therefore, about scraping off the gas bubbles and also removing "threads" of chromium by which it is understood that he means dendritic material, he is primarily interested in conducting a burnishing operation and spacing his cathode from his anode by his relatively wide spacer block.
U.S. Pat. No. 3,183,176 issued May 11, 1965 to B. A. Schwartz, Jr., discloses the electrolytic treatment or coating of a bore by use of a brush coating apparatus mounted on a drill press. The inside of the bore is acted upon by a series of centrifugally extended rotating vanes having dielectric outer covers.
U.S. Pat. No. 3,751,346 issued Aug. 7, 1973 to M. P. Ellis et al., discloses an arrangement by which a combined plating and honing procedure may be followed. In the arrangement, a plurality of honing stones are arranged to be movable into contact with the surface of the workpiece during the actual plating operation resulting in better surface characteristics, superior, it is said, to what was obtained before.
U.S. Pat. No. 3,772,164 issued Nov. 13, 1973 to M. P. Ellis et al., discloses the use of honing stones which hone the surface of a workpiece as an electrolytic coating is being applied.
U.S. Pat. No. 3,886,053 issued May 27, 1975 to J. M. Leland, discloses a method of electrolytic coating involving pulsing the current through an electrolyte containing a chromium plating solution while simultaneously performing a honing operation. It is disclosed by Leland that the honing of a chromium coating, for example, allows a high current density and faster deposition than the normal electrolytic tank process.
U.S. Pat. No. 4,125,447 issued Nov. 14, 1978 to K. R. Bachert, discloses the use of a brush attached to a movable anode within a hollow member being electroplated. The brush comprises a plurality of bristles made from plastic or other insulated material which rub against the inside surface of the tube being electroplated as the anode vibrates.
U.S. Pat. No. 4,176,015 issued Nov. 27, 1979 to S. Angelini, discloses the brushing of the surface of a series of bars as they are passed in a straight line through an anode immersed within an electroplating bath. The brushing is provided by a glass fiber brush comprising a blade having a layer of fiber scraping material compressed between side plates which is said to remove a cathodic film from the coated surface.
U.S. Pat. No. 4,210,497 issued Jul. 1, 1980 to K. R. Loqvist et al., discloses the coating of hollow members including movement inside the cavity of such members of an electrolytic solution by means of a "conveyor" which consists of a resiliently and electrically insulating material such as perforated, net-like or fibrous strip which is wound helically around a reciprocating anode. The function of the resilient electrically insulated material is to act as a conveyor of electrolyte, foam and gases which can be supplemented by forming the anode as a screw conveyor.
U.S. Pat. No. 4,595,464 issued Jun. 17, 1986 to J. E. Bacon et al., discloses the use of a so-called brush belt for continuously treating a workpiece. The brush belt is in the form of a continuous loop which passes over suitable rollers or pulleys and brings plating solution in the brush portion to the plating area. Essentially, Bacon et al. provides an absorbent belt which passes in opposition to the material to be coated.
U.S. Pat. No. 4,853,099 issued Aug. 1, 1989 to G. W. Smith discloses a so-called gap coating apparatus and process in which a relatively small elongated gap is established through which coating solution is passed at a high rate. It is said that the ultra high flow rate allows very high current densities. It is stated the process is not well suited for chromium plating, because high current densities do not increase the plating out of chromium.
U.S. Pat. No. 4,931,150 issued Jun. 5, 1990 to G. W. Smith, discloses a so-called gap-type electroplating operation in which a selected area of workpieces is coated by forming an electrode closely about such so-called gap and passing electrolytic solution through the gap at a high rate. It is stated that the ultra-high volume flow assures the removal of gas bubbles, the maintenance of low temperature and high solution pressure contact with the anode surface and a workpiece surface. It is stated that gaps approaching two and one half inches can employ the invention, but the gap would preferably be smaller, but at least 0.05 inches in width. It is stated that a fresh plating solution having a controlled temperature and no staleness is available at all times in the gap for uniform plating and while in high pressure contact with the surface of the gap. In practice, the plating solution is forced in a vertically upward direction so that any gas generated by the electrolysis in the gap migrates upwardly in the same flow direction as the plating solution is being driven and, therefore, can readily escape. It is also stated that chromium is difficult to use in the invention because chromium deposits slowly regardless of current density so that the deposition is slow and the advantages of gap plating are not fully attained.
While other processes and apparatus have, therefore, been available to remove hydrogen bubbles from cathodic coating surfaces, sever and remove dendritic material in coating processes such as the electrolytic coating of chromium and prevent depletion of the electrolytic solution and to some extent, establish a desirable coating gap between the coating electrode and the material being coated, all such prior processes have had drawbacks and none has been effective to accomplish all four or even two or three of the disclosed aims of the present invention by themselves.